When printing multi-color images, accurate registration is required between colors. Since most containers have neither accurate reference features nor stiffness, it is difficult to print multi-color images on them. Such printing normally requires multiple printing units (one for each color). Registration is difficult to maintain when a container is transferred between successive printing units. For this reason, most color images on bottles are done by applying a pre-printed label to the bottle, increasing production costs over direct printing. In some cases, such as when printing drinking cups or unfilled cans, a mandrel may be inserted into the container to achieve stiffness and registration (see for example U.S. Pat. Nos. 5,193,456 and 3,661,282). In the great majority of cases, the insertion of a mandrel to fill the container and allow registration is not possible, as inserting a mandrel requires that the container have an opening at least as large as its largest cross-section.
Flexographic printing is an ideal process for printing on thin-walled containers, as flexographic printing requires almost no pressure. Accordingly, a method and apparatus for flexographic printing on containers is highly desirable. A typical flexographic press comprises an ink supply (also referred to as an “ink fountain”), and a metering roll in contact with the ink supply. The metering roll transfers an accurately-metered amount of ink to the plate (which is mounted on a plate cylinder). The flexographic press prints on a material to be printed, usually in the form of a web, and includes an impression cylinder used to support the web. The most common form of metering roll is known as an anilox roll. An anilox roll is a hard cylinder engraved with a continuous pattern of small pits. Excess ink is removed by a doctor blade or a reverse roll, leaving ink only in the recessed areas. The flexographic plate operates in a manner similar to the common rubber stamp: the elevated areas are inked and this ink is transferred to the web. The plate is usually mounted on a thin layer of cushioning foam.
There is a need for practical systems for printing monochrome and color images directly onto containers, such as plastic and glass bottles, cans, cups, jars and the like. There is a particular need for such systems which can maintain registration between images applied by different printing units in a manner compatible with present flexographic press design.